ABSTRACT The immune system's ability to adjust the potency of its response to an external threat is exploited by most immunotherapies. Extremely successful examples are the checkpoint therapies that target the inhibitory receptors PD1 or CTLA4 to treat cancer. These immunotherapies cure some patients, but successful outcomes are still limited. Like most immunotherapies, checkpoint therapies target pathways that affect the amplitude and duration of existing immune response, but do not target the pathways that actually induce immune responses, e.g. the antigen receptor pathways in T and B cells. One reason for this is that appropriate immune responses have to stay within the narrow window between autoimmunity and immunodeficiency. Traditional treatments that block or activate cell surface receptor or enzymes are to crude and therefore frequently cause immunodeficiency or immune responses that cause detrimental side effects, such as cytokine storms and autoimmunity. Targeting antigen receptors directly requires a fine adjustment of activation thresholds, signaling amplitudes and durations. Recent findings suggest that the assembly kinetics of signaling molecules and their distribution within nanometer sized plasma membrane domains control T cell receptor (TCR) signal transduction. Targeting these mechanisms would change the probabilities of particular signaling events to occur while maintaining antigen specificity and inherent feedback mechanism. Therapies based on these principles might only slightly change T cell activation thresholds, strength and duration and result in additional or less immune responses without the detrimental side effects. The proposed research uses a combination of biochemical, structural and cutting-edge microscopy approaches to elucidate the molecular underpinnings that control the assembly dynamics and plasma membrane distribution of signaling molecules in the TCR signaling cascade. The studies will provide a comprehensive understanding of the mechanism that control the release of Zap70 kinase from the TCR into the plane of the plasma membrane to amplify and disperse antigenic stimuli. Single molecule imaging will show the first example of a kinase (i.e. Zap70) that is recruited to a receptor without intrinsic catalytic activity (i.e. the TCR) and released to encounter its substrates in spatially distinct membrane domains. Most importantly, the potential of changing T cell responses against tumor or self-antigens by modulating Zap70 conformations and thereby its interaction kinetics with the TCR will be tested in animal models for melanoma and diabetes.